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Semiconductor Unilateral Switch - Thyristor
Semiconductor unilateral switch also abbreviated as SUS, it is a two terminal, four-layer device. It will start conduction when the specific forward voltage is applied to its terminals. As we know it is four-layered, there are three junctions when the forward voltage is applied across its terminal two junctions J1 and J3 get forward biased and middle junction which is J2 gets reversed bias. J2 junction holds the current until applied voltage across its terminals reaches specific forward breakdown voltages of the device since the applied voltage across terminal reaches the forward break over voltage J2 will be broken and the device will start conducting.
It is very easy to understand the construction of two transistor equivalent circuit of SUS. Transistor equivalent circuit is given in figure 1.2 (b). Let's assume four-layers are split in such way that it looks like two transistors i.e. PNP and NPN as shown in figure 1.2 (a). Both transistors are connected in such way that the base of Q1 is connected with the collector of Q2 and base of Q2 is connected to the collector of the Q1.
To understand the working operation, Assume a transistor equivalent circuit of a SUS - thyristor connected with variable DC power supply in forwardly biased manner. As shown in figure 1.3. When VS is increased from zero VAK will also increase, SUS block conduction until VAK reaches to the VBR (Foward Biased Breakover voltage). When VAK reached to VBR there will be enough leakage current from Q1 to turn on Q2. This leakage current will cause Q2 into the saturation state, hence conduction through the device will be stared.
The voltage VAK will be decreased because it behaves like a short circuit which provides the least resistive path to current. The forward current through the saturated SUS can be found as: